I have iMac (27-inch, Mid 2011) 2.7 GHz Intel Core i5 12 GB 1333 MHz DDR3 and I have installed a 128GB SSD into the system. Formatted it for mac and it is sitting empty right now. I have the main 1TB drive still installed and working right now with the OS on it. I would like to install the OS on it and put all my other files on the 1TB drive that has everything on it including the OS Yosemite 10.10.1 . I have tried to set this up myself but have had to start over and ask for help from you fine people here. I got as far as installing the OS on the SSD and it worked fine, did not know how to remove the os from the original HHD without killing everything on the old HHD so I stopped pretending that I know more then I do. So as it sites right now 128GB SSD and 1TB HHD installed on this computer I have a time machine backup for the main HHD now.
If you've got a functioning OS on the SSD then you should be able to set this as the boot drive in System Prefs. Boot off it and then use Disk Utility to unmount the 1tb drive. Test everything with the 1tb drive unavailable. If everything works fine without it then you can remove what you want from the 1tb drive.
If you have chosen the SSD in System Preferences -> Startup Disk, and it is working, then you should have no problems removing the OS from the spinning drive.
But first, how and where did you install the SSD? In the optical drive bay with a kit is fine, in an external case is not fine.
Assuming it's internal, do this in stages.
First, boot from the SSD. Go to the 1Tb spinning drive and rename System and Library to something else. Reboot again. If this is successful you have set up your SSD properly.
Now open the terminal and type the following:
sudo chflags nohidden bin dev etc private sbin tmp usr var
In a couple of seconds, these folders should appear in the Finder window of your spinning drive. Drag them, along with Library and System, to the trash.
If you are feeling brave, empty the trash. Otherwise, reboot the computer and then empty the trash. (If it boots, these files are not in use)
You should see an extra 5-10GB of available space now.
I know this isn't exactly what you're asking for, but if you're interested, you could make your own Fusion Drive.